This invention relates generally to a composite filter circuit and more particularly to a composite filter circuit which has a low-pass filtering characteristic and high-pass or band-pass filtering characteristic and which is suitable for use in a discrete multichannel disc reproducing apparatus.
A discrete four-channel record disc system is shown in United States as U.S. Pat. No. 3,686,471. A direct wave is comprised of the sum of a pair of two channels and an angle-modulated wave. The modulated wave is angle modulating a 30 KHz carrier wave responsive to the difference between a pair of two channels. These signals are superimposed and recorded on the side walls of the disc sound groove.
In picking up and reproducing a recorded signal from this multi-channel record, there is a need for taking out the angle-modulated difference signal from the picked up signal. This modulated signal is separated from the direct-wave sum signal and then demodulated. It is to be understood that the direct-wave sum signal has a frequency band ranging from 0 to 15 KHz, while the angle-modulated difference signal has a frequency band ranging from 20 KHz to 45 KHz.
In recording on a disc with a cutting stylus or in tracing a sound groove with a pick-up stylus, cutting or tracing distortion is generally inevitable. Because of these kinds of distortions, harmonics waves of the direct-signal are produced. The harmonic components are present in the vicinity of the frequency band of the angle-modulated wave.
In order to extract ideally only the angle-modulated signal component which contains no harmonic components of the direct-wave signal, the filtering characteristic of a high-pass filter (or band-pass filter) must be made sufficiently steep for passing the angle-modulated wave signal. On the other hand, a low-pass filter for deriving the direct wave signal, is also required to have a steep filtering characteristic so as not to pass the angle-modulated carrier wave.
However, simple filters having ideal filtering characteristics, satisfying the above described requirements, have not been available. Furthermore, since the above mentioned low-pass filter and high-pass filter were heretofore provided respectively as independent circuits, they required a large number of circuit elements.